Magnetic recording medium

ABSTRACT

A magnetic recording medium having a non-magnetic support, at least one primer layer on one surface of the support, a magnetic layer on the primer layer and a back coat layer on the other surface of the non-magnetic support, in which the support has a thickness of 2 to 5 μm, the surface roughness (Ra) of the support on its surface carrying the primer layer and the magnetic layer is from 2.5 nm to 20 nm, the thickness of the primer layer is 1.5 μm or less, and the primer layer contains 2 to 30% by weight, based on the weight of all inorganic powder in the primer layer, of alumina powder having a particle size of 0.01 μm to 0.1 μm.

This application is a Divisional Application based on Continuationapplication Ser. No. 10/767,204 filed on Jan. 30, 2004 which was basedon application Ser. No. 09/749,830, filed on Dec. 28, 2000, the entirecontents of which are hereby incorporated by reference and for whichpriority is claimed under 35 U.S.C. § 120; and this application claimspriority of Application No. 11-372930 filed in Japan on Dec. 28, 1999under 35 U.S.C. § 119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium having ahigh recording capacity, a high access rate and a high transmissionrate, in particular, a magnetic recording medium for data backup.

2. Prior Art

Magnetic tapes find various applications such as audio tapes, videotapes, computer tapes, etc. In particular, in the field of tapes fordata backup, with the increase of the capacity of a hard disc whichshould be backed up, a tape having a memory capacity of several ten GBper one volume has been commercialized, and it is inevitable to increasethe capacity of the backup tape to cope with the further increase of thecapacity of the hard disc. Furthermore, it is necessary to increase arelative speed between the tape and a magnetic head to increase theaccess rate and the transmission rate.

With the magnetic tape which can cope with the increase of the memorycapacity per one volume and the increase of the travelling speed of thetape and the relative speed between the tape and the magnetic head, itis necessary to improve the touch between the tape and the magnetic headthrough the optimization of the mechanical properties of a non-magneticsupport, a primer layer and a magnetic layer as well as the increase ofa recording density through the improvement of the magnetic layer withthe increase of the magnetic properties and dispersion of aferromagnetic powder and the increase of the memory capacity through theincrease of the tape length per one volume with the reduction of thetotal thickness of the tape.

In connection with the improvement of the magnetic properties of theferromagnetic powder, a ferromagnetic metal ion powder is mainly used inplace of conventionally used metal oxide powders or cobalt-containingiron oxide powder, since the larger residual magnetization in themagnetic layer is more preferable for the increase of output. Thus, aferromagnetic iron-based metal powder having a coercive force of 120 A/m(1,500 Oe) or more is proposed (for example, JP-A-6-25702,JP-A-6-139553, etc.)

To improve the dispersion of the ferromagnetic powder, it is proposed touse a binder having a polar functional group such as a sulfonic acidgroup, a phosphoric acid group or its alkali salt, to use a lowmolecular weight dispersant together with a binder, to continuouslycarry out kneading and dispersing steps of a magnetic paint, or to add alubricant to a magnetic paint after dispersing (for example,JP-A-2-101624, JP-A-3-216812, JP-A-3-17827, JP-A-8-235566, etc.)

To improve the touch between the tape and the magnetic head so as todecrease spacing loss between them, it is proposed to smoothen themagnetic layer under conditions of a high temperature and a highpressure in a calendering step in addition to the increase of thedispersibility of the magnetic powder (for example, JP-B-1-1297,JP-B-7-60504, JP-A-4-19815, etc.)

In addition to the improvement of the properties of the magnetic layer,it is proposed to decrease the thickness of the magnetic layer to 0.6 μmor less with the provision of a primer layer between a non-magneticsupport and the magnetic layer to make the structure of the magneticrecording medium suitable for sort wavelength-recording (JP-A-5-234063).Such a magnetic recording medium has the primer layer to decreaseself-demagnetization loss and reproduction loss due to the reduction ofthe thickness of the magnetic layer and also to suppress thedeterioration of the travelling property and durability of the magneticrecording media due to the reduction of the thickness of the magneticlayer.

On the other hand, with the recent development of recording systems, itis tried to further decrease the recording wavelength. For example, thelatest digital data storage systems use the shortest recordingwavelength of 0.5 μm or less. In general, as the thickness of themagnetic layer increases, the filling amount of the magnetic powder perunit area increases, and thus the output increases. However, when aratio of the thickness of the magnetic layer to the wavelength exceeds acertain value, a demagnetizing field increases and thus the output doesnot further increase. Therefore, the thickness of the magnetic layershould be about one third (⅓) of the shortest recording wavelength.Accordingly, with the above-described latest recording systems, thethickness of the magnetic layer is reduced to 0.3 μm or less, and alsothe flatness of the surface of the magnetic layer should be improved.

In the case of the recording systems having the large capacity, thetape-travelling speed and the relative speed between the tape and themagnetic head tend to be further increased since it is necessary toincrease the access rate and the transmission rate. When thetape-travelling speed and the relative speed between the tape and themagnetic head are increased, the touch between the magnetic head and themagnetic tape becomes unstable and the output fluctuates between theentrance and the exit of a track.

To improve the flatness of the magnetic layer corresponding to thereduction of the recording wavelength, it is necessary to use anon-magnetic support having high surface smoothness. However, thenon-magnetic support having the high surface smoothness is veryexpensive and the travelling of the non-magnetic support becomesunstable since it slips or sticks to a roll when a coating layer such asthe primer layer is formed. Therefore, the productivity of the magneticrecording media deteriorates.

When the thickness of the primer layer is decreased to 1.5 μm or less todecrease the total thickness of the magnetic recording medium, theflatness of the surface of the primer layer becomes insufficient. Whenthe magnetic layer is formed on such a primer layer by a wet-on-wetmethod, minute unevenness is formed at the interface between the primerlayer and the magnetic layer. Such unevenness not only adversely affectthe writing and reading properties of the tape but also generates edgeweave at tape edges when a raw sheet of magnetic tapes is slit in aspecific width. The edge weave adversely affects the tracking of themagnetic head and thus cause the fluctuation of the output. Thisphenomenon is remarkable when the cheap non-magnetic support which hasgood travelling properties in the course of coating and low surfaceflatness.

Accordingly, when the non-magnetic support with low surface flatness isused, it is highly desired for magnetic recording tapes to cope with thedecrease of the recording wavelength through the improvement of theflatness of the surface of a magnetic recording layer, and also todecrease the fluctuation of the output through the suppression of theedge weave at the tape edges and the fluctuation of the output of themagnetic head between the entrance and the exit of the track through theimprovement of the touch between the magnetic head and the magnetictape.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a magnetic recordingmedium which increases the output and suppress the fluctuation of theoutput.

According to the present invention, there is provide a magneticrecording medium comprising a non-magnetic support, at least one primerlayer on one surface of said non-magnetic support, a magnetic layer onsaid primer layer and a back coat layer on the other surface of saidnon-magnetic support, wherein said non-magnetic support has a thicknessof 2 to 5 μm, the surface roughness (Ra) of said non-magnetic support onthe surface carrying said primer layer and said magnetic layer is from2.5 nm to 20 nm, the thickness of said primer layer is 1.5 μm or less,and said primer layer contains 2 to 30 wt. %, based on the weight of allinorganic powder in said primer layer, of alumina powder having aparticle size of 0.01 μm to 0.1 μm.

In one preferred embodiment of the magnetic recording medium of thepresent invention, the non-magnetic support has a thickness of 2.5 to4.5 μm, the primer layer has a thickness of 0.3 to 1.5 μm and a surfaceroughness (Ra) of 3 to 9 nm on its surface carrying the primer layer andthe magnetic layer, the magnetic layer has a thickness of 0.02 to 0.3μm, a coercive force of 135 to 280 kA/m and a residual magnetic fluxdensity of at least 0.18 T in the machine direction, and the back coatlayer has a thickness of 0.2 to 0.8 μm.

The present invention is based on the following findings:

When the primer layer contains the specific amount of alumina powderhaving a specific particle size, the produced magnetic recording mediumhas good short wavelength-recording characteristics and the fluctuationof the output caused by the edge weave is suppressed, even when thenon-magnetic support has low surface flatness. This effect increases,when the alumina used has a specific crystalline structure.

In addition, when the Young's modulus in the machine direction of thenon-magnetic support exceeds a specific value and the ratio of theYoung's modulus in the machine direction to that in the transversedirection of the non-magnetic support is in a certain range, thefluctuation of the output between the entrance and the exit of the trackcan be decreased. Furthermore, when the Young's modulus of thenon-magnetic support is in the specific range described above and theYoung's modulus of the coated layers consisting of the primer layer andthe magnetic layer is in the specific range, the touch between themagnetic recording medium and the magnetic head is further improved andthus the fluctuation of the output between the entrance and the exit ofthe track is further decreased.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, the magnetic recording mediumcomprises the non-magnetic support having a low surface roughness (Ra)of from 2.5 nm to 20 nm, preferably from 2.5 nm to 15 nm, morepreferably from 3 nm to 9 nm on the surface carrying the primer layerand magnetic layer, and the thickness of the primer layer is 1.5 μm orless. In such a case, the primer layer containing 2 to 30 wt. %, basedon the weight of all the inorganic powder in the primer layer, ofalumina powder having a particle size of 0.1 μm or less has low surfaceunevenness and thus the surface unevenness of the magnetic layer whichis formed on the primer layer by the wet-on-wet method can decrease,since the flowability of the primer coating composition increases. As aresult, the magnetic recording medium has the same shortwavelength-recording characteristics as those of the magnetic recordingmedia comprising the smooth non-magnetic support. Such effects aresignificant when the alumina contained in the primer layer comprisesalumina having the corundum phase.

With the magnetic recording medium comprising the primer layercontaining the alumina powder, the relationship of the Young's modulusin the machine direction of the non-magnetic support and the ratio ofthe Young's modulus in the machine direction to that in the transversedirection with the difference of the output of the magnetic head betweenthe entrance and exit of the track is studied. When the Young's modulusin the machine direction of the non-magnetic support is at least 9.8 GPa(1,000 kg/mm²) and the ratio of the Young's modulus in the machinedirection to that in the transverse direction is in the range between0.65 and 0.75, the touch between the magnetic recording medium and themagnetic head is improved and thus the fluctuation (flatness) of theoutput of the magnetic head between the entrance and exit of the trackdecreases

Hereinafter, the non-magnetic support, the primer layer, the magneticlayer and the back coat layer will be explained.

Non-Magnetic Support

In the present invention, the Young's modulus in the machine directionof the non-magnetic support is preferably at least 9.8 GPa (1,000kg/mm²), and the ratio of the Young's modulus in the machine directionto that in the transverse direction is preferably in the range between0.65 and 0.75. More preferably, the Young's modulus in the machinedirection of the non-magnetic support is at least 10.78 GPa (1,100kg/mm²), and the ratio of the Young's modulus in the machine directionto that in the transverse direction is in the range between 0.67 and0.73.

When the Young's modulus in the machine direction of the non-magneticsupport is less than 9.8 GPa, the travelling of the tape becomesunstable.

When the ratio of the Young's modulus in the machine direction to thatin the transverse direction is outside the range of 0.65 to 0.75, thefluctuation of the output of the magnetic head between the entrance andthe exit of the track may increase. This fluctuation is minimized whenthis ratio of the Young's modulus is around 0.70.

Examples of the non-magnetic support having the above properties includebiaxially orientated films of aromatic polyamide and aromatic polyimide.

The thickness of the non-magnetic support depends on the application ofthe magnetic recording media. Usually, the thickness of the support isfrom 2 to 5 μm, preferably from 2.5 to 4.5 μm. When the thickness of thesupport is less than 2 μm, the production of the film is difficult andthe tape has insufficient strength. When the thickness of the supportexceeds 5 μm, the total thickness of the tape increases so that thememory capacity per one volume decreases.

The particle size of the alumina added to the primer layer is preferably0.1 μm or less, and the amount of the alumina is preferably from 2 to 30wt. % based on the weight of all the inorganic powder in the primerlayer.

When the particle size of the alumina exceeds 0.1 μm, the effect of thealumina to improve the surface smoothness of the primer layer tends todecrease. The particle size of the alumina is preferably from 0.01 to0.1 μm, more preferably from 0.03 to 0.09 μm, particularly preferablyfrom 0.05 to 0.09 μm.

However, the above particle size does not exclude the addition ofα-alumina having a particle size of 0.1 to 0.8 μm in an amount of lessthan 3 wt. % together with the alumina having the above specificparticle size.

When the amount of the alumina added is less than 2 wt. %, the primerpaint composition has insufficient flowability. When the amount of thealumina added exceeds 30 wt. %, the unevenness of the surfaces of theprimer layer and the magnetic layer increases. The amount of the aluminaadded is preferably from 6 to 25 wt. %, more preferably from 8 to 20 wt.%, particularly preferably from 11 to 20 wt. %.

The alumina added preferably comprises one having the corundum phase,since the Young's modulus of the primer layer can be increased and thestrength of the tape is increased by the addition of the smaller amountthan σ-, θ- or γ-alumina.

The surface roughness (Ra) of the surface of the support carrying theprimer layer and the magnetic layer is preferably from 3 to 9 nm. Whenthe surface roughness (Ra) is 9 nm or less, the unevenness of thesurface of the primer layer or the magnetic layer can be small if thethickness of the primer layer is small.

When the primer layer contains the above-described amount of the aluminahaving the above particle size, the unevenness at the interface betweenthe primer layer and the magnetic layer can be suppressed so that thefluctuation of the output due to the edge weave of the tape edges can bedecreased. This effect can be enhanced when the alumina having thecorundum phase is used. In addition, the tape strength is increased.

In addition to the above alumina powder, the primer layer may containcarbon black to increase the conductivity, or non-magnetic iron oxidepowder to increase the strength of the tape.

Carbon black (CB) added to the primer layer may be acetylene black,furnace black, thermal black, etc. The carbon black has a particle sizeof 5 to 200 nm, preferably from 10 to 100 nm. When the particle size ofCB is less than 10 nm, it is difficult to disperse CB in the primerpaint composition since CB has a structure. When the particle size of CBexceeds 100 nm, the surface flatness of the primer layer or the magneticlayer deteriorates.

The amount of CB added depends on the particle size of the CB, and ispreferably from 15 to 40 wt. % of the weight of all the inorganic powderin the primer layer

When the amount of CB is less than 15 wt. %, the effect to increase theconductivity is insufficient. When the amount of CB exceeds 40 wt. %,the effect to increase the conductivity saturates.

Preferably, CB having a particle size of 15 to 80 nm is used in anamount of 15 to 35 wt. %, and more preferably, CB having a particle sizeof 20 to 50 nm is used in an amount of 20 to 30 wt. %.

The addition of CB having such a particle size can decrease the electricresistance of the primer layer so that the generation of theelectrostatic noise and the variation of the tape travelling can besuppressed.

The non-magnetic iron oxide added to the primer layer preferably has aparticle size of 0.05 to 0.40 μm, and the amount of the non-magneticiron oxide is preferably from 35 to 83 wt. % of the weight of all theinorganic powder in the primer layer.

When the particle size of the non-magnetic iron oxide is less than 0.5μm, it is difficult to disperse it uniformly. When the particle sizeexceeds 0.40 μm, the unevenness at the interface between the primerlayer and the magnetic layer increases. When the amount of thenon-magnetic iron oxide is less than 35 wt. %, the strength of theprimer film may not be sufficiently increased. When the amount exceeds83 wt. %, the strength of the primer film tends to decrease.

The Young's modulus of the coated layers consisting of the primer layerand the magnetic layer has an optimum range. When the Young's modulus ofthe coated layers is in the range between 40 and 100% of the averagevalue of the Young's moduli in the machine and transverse directions ofthe non-magnetic support, the tape has improved durability, and thetouch between the tape and the magnetic head is improved so that thefluctuation of the output of the magnetic head between the entrance andthe exit of the track is decreased.

The Young's modulus of the coated layers is preferably in the rangebetween 50 and 100%, more preferably in the range between 60 and 90% ofthe average value of the Young's moduli in the machine and transversedirections of the non-magnetic support.

When the Young's modulus of the coated layers is less than 40% of theaverage value of the Young's moduli in the machine and transversedirections of the non-magnetic support, the durability of the coatedlayers is not improved. When it exceeds 100%, the touch between the tapeand the magnetic head may deteriorate.

In the present invention, the Young's modulus of the coated layersconsisting of the primer layer and the magnetic layer is preferablycontrolled by the adjustment of the calendering conditions.

Furthermore, the Young's modulus of the primer layer is preferably from80 to 99% of that of the magnetic layer, since the primer layer can actas a cushioning layer.

Preferably, the primer layer and the magnetic layer may containlubricants having different functions.

When the primer layer contains 0.5 to 4.0 wt. % of a higher fatty acidand 0.2 to 3.0 wt. % of an ester of a higher fatty acid based on theweight of all the powder in the primer layer, the friction coefficientbetween the tape and a rotating cylinder decreases. When the amount ofthe higher fatty acid is less than 0.5wt. %, the friction coefficientmay not sufficiently decrease. When the amount of the higher fatty acidexceeds 4.0 wt. %, the primer layer is plasticized and thus losestoughness. When the amount of the ester of the higher fatty acid is lessthan 0.5 wt. %, the friction coefficient may not sufficiently decrease.When the amount of the ester exceeds 3.0 wt. %, the excessive amount ofthe ester is transferred to the magnetic layer so that the tape and therotating cylinder tends to stick each other.

When the magnetic layer contains 0.5 to 3.0 wt. % of a fatty acid amideand 0.2 to 3.0 wt. % of an ester of a higher fatty acid, the frictioncoefficient between the tape and the rotating cylinder preferablydecreases. When the amount of the fatty acid amide is less than 0.5 wt.%, the magnetic head and the magnetic layer tend to be in direct contactwith each other so that the effect to prevent seizing decreases. Whenthe amount of the fatty acid amid exceeds 3.0 wt. %, the acid amidbleeds out so that defects such as dropouts generate. When the amount ofthe higher fatty acid is less than 0.2 wt. %, the friction coefficientmay not sufficiently decrease. When the amount of the higher fatty acidexceeds 3.0 wt. %, the tape and the rotating cylinder tends to stickeach other.

In the present invention, the mutual migration of the lubricants betweenthe primer layer and the magnetic layer is not excluded.

The thickness of the magnetic layer is preferably from 0.02 to 0.3 μm,preferably from 0.02 to 0.25 μm. When the thickness of the magneticlayer is less than 0.02 μm, the output of the magnetic head is low sincethe leaking magnetic field from the magnetic layer is small. When thethickness of the magnetic layer exceeds 0.3 μm, the output of themagnetic head decreases due to the thickness loss.

Preferably, the magnetic layer has a coercive force of 135 to 280 kA/m(1,700 to 3,500 Oe) in the machine direction and a residual magneticflux density of at least 0.18 T (1,800 G) in the machine direction. Whenthe coercive force is less than 135 kA/m, the output decreases due tothe demagnetization field. When the coercive force exceeds 280 kA/m, itis difficult to write the magnetic recording medium with the magnetichead. When the residual magnetic flux density is less than 0.18 T, theoutput decreases. More preferably, the coercive force is from 160 to 240kA/m (2,000 to 3,000 Oe), and the residual magnetic flux density is from0.2 to 0.4 T (2,000 to 4,000 G).

The magnetic powder added to the magnetic layer is preferablyferromagnetic iron-based metal powder. The iron based metal is intendedto mean not only metal iron powder but also metal powder of metal ironcontaining other ferromagnetic metal such as cobalt, nickel, rare earthmetals, etc.

The ferromagnetic iron-based metal powder preferably has a coerciveforce of 135 to 280 kA/m (1,700 to 3,500 Oe) and a residual magneticflux density of 120 to 200 A·m²/kg (120 to 200 emu/g), more preferably130 to 180 A·m²/kg (130 to 180 emu/g).

The above coercive forces and residual magnetic flux densities of themagnetic layer and the ferromagnetic iron-based metal powder aremeasured using a sample vibration type magnetic flux meter in anexternal magnetic field of 1.28 MA/m (16 kOe).

The ferromagnetic iron-based metal powder used in the present inventionpreferably has an average major axis length of 0.03 to 0.2 μm, morepreferably 0.03 to 0.18 μm, particularly preferably 0.04 to 0.15 μm.When the average major axis length is less than 0.03 μm, theagglomeration force of the magnetic powder increases and thus thedispersion of the powder in the coating paint becomes difficult. Whenthe average major axis length exceeds 0.2 μm, the coercive forcedecreases and a particulate noise due to the size of the magnetic powderparticle increases.

The above average major axis length is obtained by taking a photographof the magnetic powder particles with a scanning electron microscope,measuring the major axis lengths of 100 particles, and then averagingthe measured lengths.

The ferromagnetic iron-based metal powder preferably has a BET specificsurface area of at least 35 m²/g, more preferably at least 40 m²/g, mostpreferably at least 50 m²/g.

The primer layer and the magnetic layer usually contain a binder.Examples of the binder include combinations of a polyurethane resin withat least one other resin selected from the group consisting of vinylchloride-base resins (e.g. a polyvinyl chloride resin, a vinylchloride-vinyl acetate copolymer resin, a vinyl chloride-vinyl alcoholcopolymer resin, a vinyl chloride-vinyl acetate-vinyl alcohol copolymerresin, a vinyl chloride-vinyl acetate-maleic anhydride copolymer resin,a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymerresin, etc.) and nitocellulose. Among them, the combination of thepolyurethane resin and the vinyl chloride-hydroxyl group containingalkyl acrylate copolymer resin is preferable. Examples of thepolyurethane resin include polyester polyurethane, polyetherpolyurethane, polyetherpolyester polyuretahen, polycarbonatepolyurethane, polyester-polycarbonate polyurethane, etc.

In particular, the primer layer and the magnetic layer preferablycontain a polyurethane resins having a functional group as a binder.Examples of the functional group include —COOH, —SO₃H, —OSO₂M,—(P═O)—(OM)₃, —O—(P═O)—(OM)₂ (wherein M is a hydrogen atom, an alkalimetal or an amine group), —OH, —NR¹R², —N⁺R³R⁴R⁵ (wherein each of R¹ toR⁵ is a hydrogen atom or a hydrocarbon group) or an epoxy group. Thepolyurethane resin having such a functional group can improve thedispersion of the magnetic powder.

When two or more binder resins are used, they preferably have thefunctional groups having the same polarity, in particular, —SO₃M.

The amount of the binder is usually from 7 to 50 wt. parts, preferablyfrom 10 to 35 wt. parts, based on 100 wt. parts of the ferromagneticiron-based metal powder. In particular, 5 to 30 wt. parts of the vinylchloride base resin and 2 to 30 wt. parts of the polyurethane resin arepreferably used in combination.

It is preferable to use a thermosetting crosslinking agent, whichcrosslinks the binder with bonding the functional group in the binder,along with the binder. Examples of such a crosslinking agent includediisocyanates (e.g. tolylene diisocyanate, hexamethylene diisocyanate,isophorone diisocyanate, etc.), reaction products of such isocyanateswith compounds having two or more hydroxyl groups (e.g.trimethylolpropane, etc.), condensation products of such isocyanates,and the like.

The crosslinking agent is used in an amount of 10 to 50 wt. parts,preferably 15 to 35 wt. parts, based on 100 wt. parts of the binder.

The magnetic layer may contain any conventional abrasive. The abrasiveis preferably an inorganic material having Mohs hardness of at least 6.Examples of such an inorganic material include α-alumina, β-alumina,silicon carbide, chromium oxide, ceriumoxide, α-ironoxide, corundum,artificial diamond, silicon nitride, titaniumcarbide, titaniumoxide,silicondioxide, boron nitride, etc. They may be used independently or asa mixture of two or more. Among them, alumina is preferable since it canexhibit good magnetic head-cleaning effects in a small amount.

The particle size of the abrasive depends on the thickness of themagnetic layer. The average particle size of the abrasive is preferablyfrom 0.02 to 0.4 μm, more preferably from 0.03 to 0.3 μm.

The amount of the abrasive added is preferably from 5 to 20 wt. %, morepreferably from 8 to 18 wt. %, based on the weight of the ferromagneticiron-base metal powder.

In the present invention, the magnetic layer may contain carbon black toincrease the conductivity and the surface lubrication. Examples ofcarbon black include acetylene black, furnace black, thermal black, etc.

The particle size of carbon black is preferably from 5 to 200 nm, morepreferably from 10 to 100 nm. When the particle size of carbon black isless than 5 nm, it is difficult to disperse carbon black in the magneticpaint, while when the particle size exceeds 200 nm, a large amount ofcarbon black should be added. In either case, the surface roughness ofthe magnetic layer increases so that the output decreases.

The amount of carbon black is preferably from 0.2 to 5 wt. %, morepreferably from 0.5 to 4 wt. % based on the ferromagnetic powder.

The back coat layer maybe any one used in the conventional magneticrecording media to improve the travelling properties. Usually, the backcoat layer has a thickness of 0.2 to 0.8 μm. When the thickness of theback coat layer is less than 0.2 μm, the travelling properties of themagnetic recording medium may not be sufficiently improved. When thethickness of the back coat layer exceeds 0.8 μm, the total thickness ofthe magnetic recording medium becomes too large so that the memorycapacity per one volume decreases.

The back coat layer preferably contains carbon black. Examples of carbonblack include acetylene black, furnace black, thermal black, etc.

Usually, carbon black having a smaller particle size and one having alarger particle size are used together. The carbon black having asmaller particle size has usually a particle size of 5 to 200 nm,preferably 10 to 100 nm. When this particle size is less than 5 nm, itis difficult to disperse carbon black in the back coat paint, while whenthe particle size exceeds 200 nm, a large amount of carbon black shouldbe added. In either case, the surface roughness of the back coat layerincreases to cause the embossing of the magnetic layer.

When the carbon black having a particle size of 300 to 400 nm is used inan amount of 5 to 15 wt. % of the carbon black having a smaller particlesize, the surface roughness of the back coat layer is not increased andthus the travelling properties are improved.

The total amount of the carbon black having a smaller particle size andone having a larger particle size is preferably from 60 to 98 wt. %,more preferably from 70 to 95 wt. % based on the weight of all theinorganic powder in the back coat layer.

The back coat layer has preferably a surface roughness (Ra) of 3 to 8nm, more preferably 4 to 7 nm.

The back coat layer preferably contains non-magnetic iron oxide having aparticle size of 0.1 to 0.6 μm, more preferably 0.2 to 0.5 μm toincrease the strength of the back coat layer.

The amount of the non-magnetic iron oxide is preferably from 2 to 40 wt.%, more preferably from 5 to 30 wt. % based on the weight of all theinorganic powder in the back coat layer.

A cassette tape containing the above magnetic tape installed therein hasa large capacity per one volume, and high reliability as a magneticrecording tape for backing up a hard disc drive.

EXAMPLES

The present invention will illustrated by the following Examples, whichdo not limit the scope of the present invention in any way.

In the Examples, “parts” means “parts by weight”.

Example 1

Components of Primer Paint Parts (1) Non-magnetic iron oxide powder 68(particle size: 0.11 μm × 0.02 μm) Alumina (Degree of alphatization:50%, 8 Particle size: 0.07 μm) Carbon black (particle size: 25 nm) 24Stearic acid 2.0 Vinyl chloride copolymer 10 (SO₃Na group content: 0.7 ×10⁻⁴ eq./g) Polyester polyurethane resin (Tg: 40°, 4.5 SO₃Na groupcontent: 1 × 10⁻⁴ eq./g) Cyclohexanone 25 Methyl ethyl ketone 40 Toluene10 (2) Butyl stearate 1 Cyclohexanone 70 Methyl ethyl ketone 50 Toluene20 (3) Polyisocyanate 4.5 Cyclohexanone 10 Methyl ethyl ketone 15Toluene 10

Components of Magnetic Paint Parts (1) Ferromagnetic iron-base metalpowder 100 (Co/Fe: 30 atomic %, Y/(Fe + Co): 3 atomic %, Al/(Fe + Co): 5wt. %, Ca/Fe: 0, σs: 155 A · m²/kg, Hc: 188.2 kA/m, pH: 9.4, Major axislength: 0.10 μm) Vinyl chloride-hydroxypropyl acrylate copolymer 11(SO₃Na group content: 0.7 × 10⁻⁴ eq./g) Polyester polyurethane resin 5(SO₃Na group content: 1.0 × 10⁻⁴ eq./g) α-Alumina (Av. particle size:0.2 μm) 15 Carbon black (Av. particle size: 75 nm, 2.0 DBP oilabsorption: 72 cc/100 g) Methyl phosphate 2 Palmitic acid amide 1.5n-Butyl stearate 1.0 Tetrahydrofuran 65 Methyl ethyl ketone 245 Toluene85 (2) Polyisocyanate 4 Cyclohexanone 167

The components (1) of the primer paint were kneaded with a kneader, andthe components (2) of the primer paint were added and mixed. Then, themixture was dispersed with a sand mill at a residence time of 60minutes. To the dispersed mixture, the components (3) were added andstirred followed by filtration to obtain a primer paint.

Separately, the components (1) of the magnetic paint were kneaded with akneader and the mixture was dispersed with a sand mill at a residencetime of 45 minutes. The components (2) of the magnetic paint were addedto the mixture and mixed, followed by filtration to obtain a magneticpaint.

The primer paint was applied on a support consisting of an aromaticpolyamide film (“MICTRON” (trade name) of TORAY, Thickness: 3.9 μm,Young's modulus in machine direction (MD)=11 GPa, Young's modulus inMD/Young's modulus in transverse direction (TD)=0.70) in an amount suchthat the thickness after drying and calendering was 1.0 μm to for aprimer layer. Then, on the formed primer layer, the above magnetic paintwas applied in an amount such that the thickness after orientation in amagnetic field, drying and calendering was 0.2 μm, and orientated in amagnetic field and dried to form a magnetic layer.

The orientation in the magnetic field was carried out by placing a pairof N-N opposing magnets (5 kG) before the drier, and two pairs of N-Nopposing magnets (each 5 kG) at positions which were 75 cm and 25 cmrespectively, before a finger-touch dried position in the drier. Theapplication rate was 100 m/min.

Components of Back Coat Paint Parts Carbon black (particle size: 25 nm)80 Carbon black (particle size: 370 nm) 10 Non-magnetic iron oxide(particle size: 0.4 μm) 10 Nitrocellulose 45 Polyurethane resin (havingSO₃Na groups) 30 Cyclohexanone 260 Methyl ethyl ketone 525

The components of the back coat paint were dispersed in a sand mill at aresidence time of 45 minutes. To the mixture, polyisocyanate (15 parts)was added and filtered to obtain a back coat paint. Then, the back coatpaint was applied on the other surface of the polyamide film opposite tothe magnetic layer so that the thickness after drying and calenderingwas 0.5 μm, and dried.

The magnetic sheet produced was planished with a seven-stageclalendering machine comprising metal rolls at a temperature of 100° C.and a linear pressure of 150 kg/cm. Then, the sheet was rolled around acore and aged at 70° C. for 2 hours.

Thereafter, the sheet was slit in a width of a digital audio tape (DAT)(3.8 mm), and the surface of the magnetic layer was post treated withlapping tape abrading, blade abrading and surface wiping at a travellingrate of 200 m/min. to obtain a magnetic tape. In the post-treatment, the“K 1000” lapping tape was used as the lapping tape, a carbide-tippedblade was used as the blade, and a cloth of TRAYSEE (trademark) was usedto wipe the magnetic layer surface. The travelling tension was 30 g.

The magnetic tape produced was installed in a cartridge to obtain a tapefor a computer.

Examples 2-14

A tape for a computer was produced in the same manner as in Example 1except that some conditions were changed as shown in Table 1.

Examples 15-17

A tape for a computer was produced in the same manner as in Example 1except that some conditions were changed as shown in Table 2.

The tapes were evaluated as follows:

Coating Property

In the unwinding-coating-winding process, the generation of troublessuch as waving, winding corrugations, etc. was evaluated according tothe following criteria:

A: Very good

B: Good

C: Fair

D: Bad

Surface Roughness of Support (Support Ra)

The surface roughness of the support was measured with a surfaceroughness meter (SE-3FA manufactured by KOSAKA KENKYUSHO).

The support was adhered to a semicylindrical glass having a smoothsurface with allowing the surface of the support, on which the back coatlayer would be formed, to face the glass surface. Then, the surfaceroughness of the support on which the magnetic layer would be formed wasmeasured using a stylus with a tip radius of 5 μm at a magnification of100,000 times in the vertical direction with a cut-off of 0.08 mm.

Young's Modulus of Support

A sample of the support of 10 mm in width and 150 mm in length wasprovided. Then, a load-elongation curve was recorded with an INSTRONtype universal tensile meter, and a Young's modulus in MD (Y_(MD), unit:GPa) and that in TD (Y_(TD), unit GPa) were calculated.

The sample was pulled with a chuck distance of 100 mm at a pulling rateof 20 mm/min. and the Young's modulus was calculated from a load at 0.3%elongation in the recorded chart.

Young's Modulus of Magnetic Layer

A sample of the magnetic tape of 3.8 mm in width and 150 mm in lengthwas provided, and a load-elongation curve was recorded and a Young'smodulus was calculated in the same manner as in the measurement of theYoung's modulus of the support.

Edge Weave

A tape of 450 mm in length was extended between a pair of guides withflanges under a tension of 0.05 (N), and the tape edge position was readfrom the above with an edge position sensor. The edge positions wereread at an interval of 0.25 mm along 250 mm of the tape base edge. Fromthe position data obtained in the first 50 mm length, a regression linewas obtained. Then, the deviation of each position date from theregression line was measured to obtain the maximum deviation.

Next, the starting point was shifted by 0.25 mm, and the position datawere read along the length of 50 mm in the same manner as above and themaximum deviation was obtained. These procedures were repeated, and themaximum deviations along the length of 250 mm were averaged and used asthe measure of edge weave.

Flatness

A magnetic tape was recorded at a recording wavelength of 0.67 μm with aDDS-4 drive (manufactured by Hewlet-Packard), and the recorded signalwas reproduced with the same drive. The outputs were recorded at 25points from the entrance to the exit of each trace of an envelopewaveform, and averaged to obtained the averaged output (AVR).

From the AVR, the maximum output (Max) and the minimum output (Min), aflatness was calculated according to the following equations:

Maximum value (dB)=20×log(Max/AVR)

Minimum value (dB)=20×log(Min/AVR)

Flatness (dB)=Maximum value−Minimum value

The results are shown in Table 1 TABLE 1 Example No. 1 2 3 4 SupportAramid Aramid Aramid Aramid Thickness (μm) 3.9 3.9 3.9 3.9 Y_(MD) (GPa)11.0 11.3 10.6 11.0 Y_(TD) (GPa) 15.5 15.0 16.3 15.5 Y_(MD)/Y_(TD) 0.700.75 0.65 0.70 (Y_(MD) + Y_(TD))/2 (GPa) 13.3 13.2 13.5 13.3 Support Ra(nm) 6.0 6.0 6.0 2.5 Powder in primer layer AL1¹⁾ (wt. %) 8 8 8 8Crystal structure²⁾ Cor. Cor. Cor. Cor. Percentage of 50 50 50 50alphatization (%) AL2³⁾ (wt. %) 0 0 0 0 Carbon black (wt. %) 24 24 24 24Iron oxide (wt. %) 68 68 68 68 Linear pressure in 1470 1470 1470 1470calendering (N/cm) t1⁴⁾ (μm) 0.2 0.2 0.2 0.2 t2⁵⁾ (μm) 1.0 1.0 1.01.00.5 BC⁶⁾ (μm) 0.5 0.5 0.5 5.6 Tape thickness (μm) 5.6 5.6 5.6 Coatingproperty A A A B Surface roughness (Ra) 2.2 1.9 2.0 1.7 of magneticlayer (nm) Ratio of Young' modulus⁷⁾ 11.0 12.5 11.0 11.0 (%) 83 95 81 83Edge weave (μm) 2.0 2.3 2.0 2.0 Flatness (dB) 1.2 1.9 1.7 1.4 Output AVR(%) 100 92 95 100 Example No. 5 6 7 8 Support Aramid Aramid AramidAramid Thickness (μm) 3.6 3.9 3.9 3.9 Y_(MD) (GPa) 11.0 11.3 11.0 11.3Y_(TD) (GPa) 15.5 15.0 15.1 15.5 Y_(MD)/Y_(TD) 0.70 0.70 0.73 0.73(Y_(MD) + Y_(TD))/2 (GPa) 13.3 13.2 13.1 13.3 Support Ra (nm) 6.0 6.09.0 9.0 Powder in primer layer AL1¹⁾ (wt. %) 5 16 13 13 Crystalstructure²⁾ Cor. Cor. Cor. Cor. Percentage of 50 50 65 80 alphatization(%) AL2³⁾ (wt. %) 0 0 0 0 Carbon black (wt. %) 28 20 24 24 Iron oxide(wt. %) 68 64 63 63 Linear pressure in 1470 1180 1470 1470 calendering(N/cm) t1⁴⁾ (μm) 0.2 0.2 0.2 0.2 t2⁵⁾ (μm) 1.3 1.0 1.0 1.0 BC⁶⁾ (μm) 0.50.5 0.5 0.5 Tape thickness (μm) 5.6 5.6 5.6 5.6 Coating property A A A ASurface roughness (Ra) 1.8 2.5 2.8 2.8 of magnetic layer (nm) Ratio ofYoung' modulus⁷⁾ 11.0 12.5 12.2 12.5 (%) 83 94 93 94 Edge weave (μm) 2.02.5 2.3 2.5 Flatness (dB) 1.4 1.9 1.7 1.8 Output AVR (%) 98 92 94 90Example No. 9 10 11 12 Support Aramid Aramid Aramid Aramid Thickness(μm) 3.9 3.9 3.9 3.9 Y_(MD) (GPa) 11.0 11.0 11.0 11.0 Y_(TD) (GPa) 16.416.4 16.4 15.5 Y_(MD)/Y_(TD) 0.67 0.67 0.67 0.70 (Y_(MD) + Y_(TD))/2(GPa) 13.7 13.7 13.7 13.3 Support Ra (nm) 9.0 9.0 9.0 11.0 Powder inprimer layer AL1¹⁾ (wt. %) 13 13 13 20 Crystal structure²⁾ Cor. Mix.Cor. Cor. Percentage of 30 0 99 50 alphatization (%) AL2³⁾ (wt. %) 0 0 00 Carbon black (wt. %) 24 24 24 24 Iron oxide (wt. %) 63 63 63 56 Linearpressure in 1470 1960 1470 1470 calendering (N/cm) t1⁴⁾ (μm) 0.2 0.2 0.20.2 t2⁵⁾ (μm) 1.3 1.0 1.0 1.0 BC⁶⁾ (μm) 0.5 0.5 0.5 0.5 Tape thickness(μm) 5.6 5.6 5.6 5.6 Coating property A A A A Surface roughness (Ra) 2.52.2 2.7 3.0 of magnetic layer (nm) Ratio of Young' modulus⁷⁾ 11.5 12.312.5 12.6 (%) 84 90 91 95 Edge weave (μm) 2.9 3.2 2.6 2.3 Flatness (dB)1.4 2.0 1.7 1.4 Output AVR (%) 79 74 88 86 Example No. 13 14 SupportAramid Aramid Thickness (μm) 3.9 3.9 Y_(MD) (GPa) 9.5 11.0 Y_(TD) (GPa)15.5 15.5 Y_(MD)/Y_(TD) 0.61 0.70 (Y_(MD) + Y_(TD))/2 (GPa) 12.5 13.3Support Ra (nm) 6.0 6.0 Powder in primer layer AL1¹⁾ (wt. %) 8 8 Crystalstructure²⁾ Cor. Cor. Percentage of 50 50 alphatization (%) AL2³⁾ (wt.%) 0 0 Carbon black (wt. %) 24 24 Iron oxide (wt. %) 68 68 Linearpressure in 1470 2940 calendering (N/cm) t1⁴⁾ (μm) 0.2 0.2 t2⁵⁾ (μm) 1.01.0 BC⁶⁾ (μm) 0.5 0.5 Tape thickness (μm) 5.6 5.6 Coating property A ASurface roughness (Ra) 2.2 1.8 of magnetic layer (nm) Ratio of Young'modulus⁷⁾ 11.0 13.8 (%) 88 104 Edge weave (μm) 2.0 2.3 Flatness (dB) 2.83.0 Output AVR (%) 78 72Notes:¹⁾α-Alumina having a particle size of 0.07 μm.²⁾Crystal structure: Cor.: Corundum, Mix.: Mixture³⁾α-Alumina having a particle size of 0.19 μm (Percengate ofalphatization: 99%)⁴⁾Thickness of a magnetic layer⁵⁾Thickness of a primer layer⁶⁾Thickness of a back coat layer⁷⁾(Young's modulus of coated layers) × 100/[(Y_(MD) + Y_(TD))/2]Notes:See the Notes for Table 1.

TABLE 2 Example No. 15 16 17 Support Aramid Aram Aramid Thickness (μm)3.9 id 3.3 Y_(MD) (GPa) 11.0 3.9 11.0 Y_(TD) (GPa) 15.5 11.0 15.5Y_(MD)/Y_(TD) 0.70 15.5 0.70 13.3 (Y_(MD) + Y_(TD))/2 (GPa) 13.3 0.706.0 Support Ra (nm) 1.0 13.3 6.0 Powder in primer layer AL1¹⁾ (wt. %) 80 8 Crystal structure²⁾ Cor. — Cor. Percentage of 50 — 80 alphatization(%) AL2³⁾ (wt. %) 0 8 0 Carbon black (wt. %) 28 24 24 Iron oxide (wt. %)68 68 68 Linear pressure in 1470 1470 1470 calendering (N/cm) t1⁴⁾ (μm)0.2 0.2 0.2 t2⁵⁾ (μm) 1.0 1.0 3.0 BC⁶⁾ (μm) 0.5 0.5 0.5 Tape thickness(μm) 5.6 5.6 7.0 Coating property D B A Surface roughness (Ra) 1.5 3.02.8 of magnetic layer (nm) Ratio of Young' modulus⁷⁾ 11.0 12.0 11.0 (%)83 90 76 Edge weave (μm) 2.5 4.3 2.0 Flatness (dB) 2.2 2.5 1.2 OutputAVR (%) 98 65 99 Remarks 8)Notes:¹⁾ to ⁷⁾See the Notes ¹⁾ to ⁷⁾ for Table 1.8) The tape having the specified length could not be installed in thecassette.

As can be seen from the results of Examples 1-14 and Examples 15-17reported in Tables 1 and 2, when the non-magnetic support having lowsurface flatness on the surface to carry the primer and magnetic layers,which is cheap and achieves a high productivity of magnetic tapes, isused, the fluctuation of the output due to the edge weave is suppressed,and also the fluctuation (flatness) of the output from the entrance tothe exit of a track is suppressed according to the present invention inwhich the primer layer has a thickness of 1.5 μm or less and contains 2to 30 wt. %, based on all the inorganic powder, of an alumina having aparticle size of 0.1 μm or less, in particular, alumina comprising onehaving the corundum phase, the thickness of the non-magnetic support isfrom 2 to 5 μm, the Young's modulus in the machine direction of thesupport is at least 9.8 GPa (1,000 kg/mm²) and the ratio of the Young'smodulus in the machine direction to that in the transverse direction ofthe support is from 0.65 to 0.75.

Furthermore, when the Young's modulus of the coated layers consisting ofthe primer and magnetic layers is from 40 to 100% of that of thenon-magnetic support, the fluctuation (flatness) of the output from theentrance to the exit of a track is further suppressed.

1. A magnetic recording medium comprising: a non-magnetic support; andat least one magnetic layer on one surface of said non-magnetic support,wherein said magnetic recording medium has a thickness of 5.6 μm or lessand an edge weave of 3.2 μm or less.
 2. The magnetic recording mediumaccording to claim 1, wherein said magnetic layer has a coercive forceof 135 to 280 kA/m in a machine direction.
 3. The magnetic recordingmedium according to claim 2, wherein said magnetic recording medium hasan edge weave of 2 μm or less.
 4. The magnetic recording mediumaccording to claim 3, wherein said magnetic layer has a coercive forceof 160 to 240 kA/m in a machine direction.
 5. The magnetic recordingmedium according to claim 3, wherein said magnetic layer has a thicknessof 0.2 μm or less.
 6. The magnetic recording medium according to claim3, wherein said non-magnetic support has a thickness of 2 to 5 μm. 7.The magnetic recording medium according to claim 3, which furthercomprises a back coat layer on the other surface of said non-magneticsupport.
 8. A magnetic recording medium comprising: a non-magneticsupport; at least one primer layer on one surface of said non-magneticsupport; and a magnetic layer on said primer layer, wherein saidmagnetic recording medium has a thickness of 5.6 μm or less, and saidmagnetic recording medium has an edge weave of 3.2 μm or less.
 9. Themagnetic recording medium according to claim 8, wherein said edge weaveis 2.0 μm or less.
 10. The magnetic recording medium according to claim9, wherein said magnetic layer has a thickness of 0.2 μm or less. 11.The magnetic recording medium according to claim 9, wherein saidmagnetic layer has a coercive force of 135 to 280 kA/m in a machinedirection.
 12. The magnetic recording medium according to claim 9,wherein said primer layer has a thickness of 1.0 μm or less.
 13. Themagnetic recording medium according to claim 9, wherein saidnon-magnetic support has a thickness of 2 to 5 μm.
 14. The magneticrecording medium according to claim 9, which further comprises a backcoat layer on the other surface of said non-magnetic support.
 15. Amagnetic recording medium comprising: a non-magnetic support; at leastone primer layer on one surface of said non-magnetic support; and amagnetic layer on said primer layer, wherein said magnetic layer has athickness of 0.3 μm or less, said primer layer has a thickness of 1.3 μmor less, and said magnetic recording medium has an edge weave of 3.2 μmor less.
 16. The magnetic recording medium according to claim 15,wherein said edge weave is 2.0 μm or less.
 17. The magnetic recordingmedium according to claim 15, wherein said magnetic layer has athickness of 0.2 μm or less.
 18. The magnetic recording medium accordingto claim 15, wherein said primer layer has a thickness of 1.0 μm orless.
 19. The magnetic recording medium according to claim 18, whereinsaid magnetic layer has a coercive force of 135 to 280 kA/m in a machinedirection.
 20. The magnetic recording medium according to claim 18,wherein said primer layer contains carbon black and at least onenon-magnetic metal oxide selected from the group consisting of aluminaand iron oxide.
 21. The magnetic recording medium according to claim 18,wherein said non-magnetic support has a thickness of 2 to 5 μm.
 22. Themagnetic recording medium according to claim 18, which further comprisesa back coat layer on the other surface of said non-magnetic support.